1. Field of the Invention
This invention concerns screening apparatus for separating on a size basis solid components of a pulp slurry. More particularly, the invention concerns such apparatus suitable for use as a "headbox screen," for screening papermaking fiber slurry immediately before it is distributed by the headbox to a papermaking machine.
2. Description of the Prior Art
Screening of paper fiber slurry immediately before it is applied to the papermaking machine requires screening apparatus with a high throughput of accepts to handle the large volume of flow required by a modern high speed paper machine, this with a low pressure drop, generally not over 6 p.s.i. This should be accompanied by a low reject rate, as the purposes of the headbox screen are primarily "de-floccing" of the stock and to eliminate the few undesirables or "junk" that may have escaped the pre-cleaning stages, these having generally sorted out most of the non-desirable fibers and junk.
In the past, screening apparatus of the so-called "foil" type such as shown in U.S. Pat. No. 2,975,899, wherein an open or "cage" rotor carries screen length foil bars that produce the requisite pulsation for effective screening, have been found most satisfactorily to possess these characteristics, along with desirably low horsepower requirements, and have been generally used for the purpose. However, they have a drawback in that the foils passing the screen outlet area produce large regular pulses that are transmitted through the stock as it passes to the papermaking machine. These pulses, added to those from the preceding fan pump which are often transmitted through the screen, have been an increasing cause for concern to paper manufacturers, because they cause wave or ripple defects in the forming paper.
Experiments have shown that screening apparatus having a drum rotor with a bumped surface providing the pulsing, as disclosed in U.S. Pat. No. 3,363,759, applied as a headbox screen, does not generate pulses that are transmitted to any harmful extent to the headbox and furthermore is effective in suppressing further transmission of pulses from the fan pump. However, such apparatus is designed primarily as a "sorter," for screening in the pulp mill fiber slurry of relatively high consistency compared to that of papermaking furnish. Its commercial embodiments do not have the high throughput capacity at low pressure drop with low rejects to accepts ratio desirable in a headbox screen.